Abstract: A series of experiments were carried out to study the crystallizing conditions of Mo-Fe protein of Azotobacter vinclandii nitrogenase. Needle-like crystals of Mo-Fe protein were obtained, with crystal size fluctuated inthe range of 2-2.5×25-65μ. The conditions required for crystallization were notvery strict. The molecular weight of Mo-Fe protein was estimated by SDS gel electrophoresisand by amino acid analysis to be approximately 250000. Only one type of subunit wasobserved; in the Mo-Fe protein acidic amino acid was predominant. It was found that oxygen caused the needle-like crystals to transform into hair-like crystals which may aggregate somewhere further into bundles. The latter underanaerobic condition could last for longer time. It was also demonstrated that duringthe gel electrophoresis the formation of a higher molecular weight polymer was depend-ent on the presence of oxygen.